Polymer light emitting diode

ABSTRACT

Device adapted for efficient light emission including a plurality of component layers of which a first outer layer is adapted for electron injection, a second opposing outer layer is adapted for hole injection, and plural intermediate layers arranged therebetween for charge semi-conduction. The intermediate layers include at least one semi-conducting polymer adapted for hole transport and/or electron blocking, and at least one semi-conducting polymer adapted for hole blocking and/or election transport. The at least one semi-conducting polymer adapted for electron transport and/or hole blocking comprises polymer selected from a nitrogen and/or sulphur containing polymer which is partially or substantially conjugated.

The present invention relates to novel polymer light emitting diodes,components and uses thereof, to a process for the production thereof anda method for light emission. More particularly the present inventionrelates to novel polymer light emitting diodes adapted for enhancedefficiency light emission, novel semi-conducting polymer components anduses thereof in displays and the like, to a process for the productionthereof and a method for light emission.

Illuminated displays have been in existence for some years now and areadvantageous in a wide range of applications. Nevertheless shortcomingsare continually being addressed by development of improved illuminationtechniques. For example cathode ray tubes are currently in operation inapplications where their high power consumption, bulk and weight areacceptable. In less demanding applications liquid crystal displays areemployed which operate by reflective rather than light emissive means.Such display technologies suffer problems of limited viewing angle, poorcontrast and the like.

Semi-conduction light emitting diodes are known, employing conventionalinorganic semi-conducting materials for light emission purpose. Theseprovide an excellent viewing angle and contrast, but limited range ofcolours. Inorganic semi-conductors are generally crystalline materialsleading to complicated manufacture and limited area of devices which maybe made from them.

Light emitting diodes (LED's) comprise two electrode layers,respectively a hole injecting layer and an electron injecting layer,typically comprising highly doped semi-conductor, metallic or ionicsheet layers, having an inorganic semi-conductor layer therebetweenwhich serves to conduct holes and electrons to a region between bothelectrodes at which photon emission occurs.

LEDs however are limited in the range of inorganic semi-conductingmaterials available and the wave length range of light in which theyemit, commonly the infra-red range.

Efforts to modify LEDs by use of conjugated semi-conducting polymershave been made, for example, as disclosed in Holmes A. B. et alSynthetic Metals 55-57 (1993) 4031-4040 “Photoluminescence andelectroluminescence in conjugated polymeric systems”, however as yet ithas not been possible to equal the external quantum efficienciesachieved with inorganic LEDs. A common arrangement employs a singe layerof polyparaphenylenevinylene (PPV) as semi-conducting polymer, which hasbeen found to give an external quantum efficiency defined as number ofphotons emitted in the form of light detected outside the device, perelectron flowing through the device of the order of 0.001% for analuminium electron injecting contract layer, and of the order of 0.01%for a calcium electron injecting contact layer.

Light emission in organic conjugated polymeric materials may occur bythe process of photo luminescence or electro luminescence, as indicatedin FIG. 1 and FIG. 2.

It will be apparent that photo luminescence may take place by photoexcitation of a polymer, without need for charge conduction, for examplephotoluminescence has observed for PPV, with emission of light at alonger wavelength (sometimes referred to as the Stokes shift) than thatabsorbed. Luminescence efficiency by radiative decay of the singletexcition can be reduced by a variety of competing non-radiative decayprocesses.

Electroluminescence is elcetrically inducded by light emission. Incontract to photoluminescence, it results from respective electron andhole injection causing excitation and negative and positive polaronformation. Coincidence of a negative and positive polaron in aluminescent material causes singlet exciton formation with emission oflight. Reduced luminescence efficiency may take place as mentioned forphotoluminescence, by migration of polarons to a “trap” or “quench”whereby energy is lost-radiatively, or as a result of the proximity ofelectrodes.

Doping of semi-conducting polymers to improve their semi-conductingbehaviour is not wholly successful due to occurrence of phaseseparation, need for energising the dopant and the like.

Despite the poor efficiency of polymeric LED's, they have highlysignificant potential advantages in terms of their processability andability to be deposited over large areas as high quality, robust and/orflexible thin films, for example enabling construction of flexible, verylarge area LED's, inherent radiative decay properties, emission rangecovering the whole range of the spectrum, and chemical tailoring ofpolymer materials to realise desired properties, to name but a few.

According there is a need for an LED and for improved polymericsemi-conductors having the advantages and versatility available withpolymeric semi-conductors, however having improved brightness andexternal quantum efficiencies greater than currently available, forexample of the order of 1000 Cd/m² and 0.5% respectively and more,thereby meeting commercial requirements.

We have now surprisingly found that it is possible to provide asemi-conducting polymer LED meeting these requirements in admirablemanner. In particular we have found that in provision of such LEDs, thearrangement of a given semi-conducting material comprised in an LED maydetermine the overall external quantum efficiency of the LED. Moreoverwe have found that the semi-conducting ability of certain materials maybe improved by careful selection of synthetic techniques.

In a first embodiment of the invention there is provided in its broadestaspect a device adapted for light emission comprising a plurality ofcomponent layers of which a first outer layer is adapted for electroninjection, a second opposing outer layers is adapted for “hole”injection, and one or more intermediate layers arranged therebetween areadapted for charge semi-conduction wherein the intermediate layer(s)comprise at least one semi-conducting polymer adapted for electrontransport and/or hole blocking, and at least one semi-conducting polymeradapted for hole transport and/or electron blocking wherein the at leastone semi-conducting polymer adapted for electron transport and/or holeblocking comprises polymer selected from a nitrogen and/or sulphurcontaining polymer which is partially or substantially conjugated.Preferably the at least one semi-conducting polymer adapted for electrontransport and/or hole blocking is selected from a conjugated polycyclicin which at least one nitrogen and/or sulphur is a heteroatom comprisedwithin a conjugated heterocyclic system. By this means the inventionprovides a mechanism which resembles the balanced injection and balancedtransport of electrons and holes. Preferably the device is characterisedby an attractive external quantum efficiency, as hereinbefore defined,and an attractive brightness, measured as Cd/m². Preferably the devicecomprises intermediate layer(s) comprised of one or more semi-conductingpolymers, the type, purity, concentration and layer thickness whereofare adapted for efficient electron and hole transport in relativemanner. Preferably the device comprises semi-conducting polymer ofchemical and physical nature adapted for electroluminescence, by polaronformation, migration, coincidence and decay in manner that at least onephoton of radiation emission is emitted from the device per 400electrons injected into the device. It is a particular advantage of theinvention that the migration and coincidence of electrons and “holes”may be manipulated, whereby a boundary region for coincidence thereofmay be positioned relative to the first and second outer layers in anemissive region in manner to provide enhanced brightness and/or externalquantum efficiency. Without being limited to this theory, it would seemthat the positioning of the boundary region is a function of therespective degrees of transport of electrons and holes within theintermediate layer(s).

Preferably a device of the invention is characterised by an externalquantum efficiency of at least 0.1%, more preferably at least 0.2%, mostpreferably at least 2.0%. Preferably a device of the invention ischaracterised by a brightness of at least 100 Cd/m², more preferably atleast 500 Cd/m², most preferably at least 2000 Cd/m².

Reference herein to a component layer is to a substantially uniformdiscrete layer of a material, the properties of which are suited to thefunction of the layer. Accordingly it will be apparent that individuallayers are distinguished by nature of component material, which maycomprise a plurality of chemical entities present as a physical orchemical mixture.

It will be appreciated that component layers are provided withsubstantially continuous interface therebetween by suitable means asknown in the art, for example by bonding, contact curing and the likewith use of co-extrusion, spin or dip coating, electro vacuum depositionprocess and the like.

Reference herein to semi-conducting layer(s) is to component layers ofelectron and “hole” transporting materials respectively, present ascomponent layers. These materials favour the transport of negative andpositive charge respectively, are also known an n-type and p-typematerials. Reference herein to electrons and holes is to negativepositive charge carriers respectively, as known in the art, also knownas negative and positive polarons.

Reference herein after to a junction is to a boundary region ofcoincidence of negative and positive charge carriers as hereinbeforedefined.

Preferably a junction comprises a substantially continuous planar regionthrough the device of the invention, substantially co-planar with thecomponent layers thereof. The junction is preferably of uniformthickness. The junction may coincide with the interface betweencomponent layers or may be comprised within one, or bridging two,component layers. Preferably the junction coincides with or bridges theinterface between two semi-conducting layers which are of differentcharge semi-conduction nature i.e., one of which is electrontransporting and the other of which is hole transporting. It is aparticular advantage that the position of the junction, which representsthe emissive region, in a particular layer or on the interface betweentwo layers provides a means of controlling emission colour, for examplethe characteristic emissive colour of one of two layers, or of bothlayers in the form of two colour emission. Preferably a precise junctionfor coincidence of transported charge is obtained, wherein eachconducting layer provides uniform and efficient transport, for theentire layer thickness thereof. This provides a novel means forimproving the efficiency and brightness of devices made from existingluminescent polymers, or from novel improved efficiency luminescentpolymers alike. Accordingly there is provided according to the presentinvention a device as hereinbefore defined wherein the junction positionmay be influenced with resulting property change, and means forinfluencing the junction position.

Preferably at least one charge semi-conduction layer, or a componentthereof, is capable of light emission by luminescence. Preferably theemissive layer(s) or component thereof are substantially coincident withthe junction of coincidence of charge carriers. More preferably the oreach charge semi-conduction layer is substantially emissive.

The junction position may be influenced by any suitable means.Preferably the junction position is a function of the nature ofsemi-conducting polymers employed and of the respective thickness ofsemi-conducting layers employed. It will be appreciated that byappropriate selection of these parameters the respective transport ofpositive and negative charge carriers (polarons) through thesemi-conducting layers may be controlled in manner that sufficientcharge coincides at a junction as hereinbefore defined to achieve theexternal efficiency as hereinbefore defined.

The ratio of thickness may be selected by those skilled in the art, forexample by calculation having regard to whether the respectivesemi-conductors are emissive or non-emissive and the degree of emission,i.e., the quantum efficiency, thereof. Preferably the ratio of thicknessare in the region of 0.1-10, more preferably 0.15-9, most preferably0.3-5, in each case the least thickness layer representing the leastefficient charge carrier transporter.

The ratio of thickness may also suitably be determined by experiment, bymeans of monitoring the brightness and efficiency ofelectro-luminescence for different ratios of thickness of componentsemi-conductor layers. For example, by determination of (degree of)luminescence achieved with a device having one semi-conducting layerunder test, in combination with a second control semi-conducting layer,only one of each which semi-conducting layers is luminescent, chemicaland physical variations of the polymer under test may be made.

Total semi-conducting layer thickness is suitable selected in the rangeof from 10 to 200 nm, preferably from 30 to 150 nm. Two low a thicknessrisks entering the range of errors of polymer thickness, wherebyshort-circuiting may occur at “bald” regions or “pinholes” havingsubstantially no polymer. Excessive thickness requires high voltageoperation with resultant power efficiency. Total layer thickness maytherefore be selected according to requirements of robustness, cost andquality.

A device as hereinbefore defined may comprise one or more electrontransporting and one or more hole transporting layers, preferablycomprises one electron transporting and one hole transporting layer.

A component layer as hereinbefore defined adapted for electron injectionis suitable comprised of any suitable electron injecting material forexample as known in the art, preferably comprises any suitable metal,alloy or semi-conductor such as calcium, magnesium, gold, aluminium andthe like optionally as an admixture with suitable agents.

It is a particular advantage that the present invention enables theselection of aluminium as electron injection layer and surprisingly giveefficient devices.

A component layer as hereinbefore defined adapted for hole injection issuitably comprised of any suitable hole injecting material for exampleas known in the art, preferably comprises any suitable metal, alloy orsemi-conductor such as indium tin oxide (ITO), tin oxide or othertransparent conductor, PEDOT, polyaniline or like polymer, gold and thelike, optionally as an admixture with suitable agents.

A component layer as hereinbefore defined adapted for electron transportis suitably comprised of any n-type conducting material for example asknown in the art but comprises at least one nitrogen and/or sulphurcontaining polymer as hereinbefore defined which optionally partly orsubstantially conjugated, preferably comprises polypyridine (Ppy),polyalkylpyridines, polypyrimidines, polyalkylpyrimidines,polythiazoles, polyalkylthiazoles, derivatives such as fluorinatedderivatives, analogues and functional equivalents thereof. Such materialmay be present in pure form, doped or undoped, protonated orunprotonated, oxidised or reduced or together with suitable agents, forexample may be doped by grafting or mixing for property enhancement.

A component layer as hereinbefore defined adapted for hole transport issuitably comprised of any known p-type conducting material for exampleas known in the art, but which comprise any suitable conjugated organicmolecule, dye or dye-doped polymer system, preferably comprisespolyparaphenylenevinylene (PPV),poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV),cyano PPV, poly(p-phenylene), poly(alkylthiophene), derivatives,monomers, oligomers, analogues and functional equivalents thereof. Suchmaterial may be present in pure form, doped or undoped, protonated orunprotonated, oxidised or reduced or together with suitable agents, forexample may be doped by grafting or mixing for property enhancement.

Preferably semi-conducting polymers are substantially uncontaminated byelectron and/or “hole” quenching or trapping species which may be reducetheir transmission efficiency. For example electron transport and/orhole blocking semi-conducting polymer, preferably polypyridine is mostadvantageously employed in high purity, preferably in substantiallycation free form, wherein cations deriving from the polymerisationreagent employed or the preparation of such reagent are substantiallyabsent as synthetic residue from the polymer structure. More preferablythe semi-conducting polymer is prepared by the reaction of precursormonomers and/or oligomers in the presence of a zero-valent chelatingmetal reagent and a salt of a cation, wherein the zero-valent chelatingmetal is present in stoichiometric excess to the cation.

Component materials as hereinbefore defined advantageously arecharacterised by a life time suited to the purpose for which the deviceis adapted. For example aluminium is characterised by a particularbeneficial life time. Polymers are advantageously resistant tophoto-oxidation.

The advantages of the present invention, in terms of high efficiencyand/or brightness can be enhanced with use of electrodes of relativelyhigh work function and hence good stability.

A device as hereinbefore defined may comprise any further supporting,sealing or protective layers and the like. Preferably a device ashereinbefore defined comprises a transparent rigid or flexible supportlayer such as quartz or glass or suitable synthetic equivalent such aspolymeric substrates on which the device is constructed or onto whichthe constructed device is transferred, whereby the integrity anduniformity thereof is preserved. A support layer may be used during theconstruction of the device, onto which either outer layer may bedeposited as desired, and intermediate and opposing layers subsequentlyapplied, and a second layer to be used during use applied to eitherouter layer as desired.

In a further aspect of the invention there is provided the novel use ofa semi-conducting polymer in a device as hereinbefore defined. In aparticular advantage of the invention it has found that the nitrogenand/or sulphur containing, optionally partially or substantiallyconjugated polymers as hereinbefore defined are characterised byexcellent electron transporting properties, given n-type conduction andimproved external quantum efficiency when used as electron transportinglayer of an LED or a device as hereinbefore defined. Moreover it hasbeen found that the efficiency thereof may be enhanced by enhancing thesynthetic quality thereof. Preferably a semi-conducting polymer for useas an n-type conducting layer in a device as hereinbefore definedcomprises polypyridine, more preferably comprises polypyridine which issubstantially free of quenches or traps for charge carriers.

In a further aspect of the invention there is provided a process for thepreparation of a nitrogen and/or sulphur containing polymer ashereinbefore defined, for example polypyridine, and the polymerobtained, by polymerisation with use of a substantially purepolymerisation reagent, for example a zero-valent nickel polymerisationreagent substantially free of salts of cations for example Zn fromZnCl₂. The polypyridine obtained by this process is found to provideexcellent purity product polymer.

In a further aspect there is provided an electron transporting and/orhole blocking polymer for use as an n-type semi-conductor, optionally ina device as hereinbefore defined, comprising a nitrogen and/or sulphurcontaining polymeric material as hereinbefore defined which ispreferably substantially free of charge carrier quenching or trappingmoieties. More preferably an electron transporting polymeric material issubstantially free of materials capable of trapping negative charge orquenching luminescence.

In particular advantage of this aspect of the invention, a highlyefficient electron or hole transporting polymeric material may be usedin desired purity or thickness to enable the variation of relativedegree of electron and hole transport in a semi-conducting device.

In a further aspect of the invention there is provided the use of adevice as hereinbefore defined as a light emitting display (LED) of anydesired surface area and for portable or fixed purpose. A display maycomprise a single continuous device or a mosaic of devices depending onthe size and nature thereof. It is a particular advantage of theinvention that a device of the invention is characterised by highquality for any application.

Preferably a device is used for display purposes, such aselectroluminescent TV or computer screens, back lighting of liquidcrystal displays such as in watches and the like, very large areadisplays such as public information boards in airports and the like,small displays such as for household electrical goods includingcalculators, washing machines and the like, flexible displays, head-up(virtual reality) displays for training, entertainment purposes and thelike, for example for aircraft pilot, road user training and the like;for improved efficiency applications such as for lighting of largeareas, preferably by means of panelled low intensity (low brightness)wall or ceiling lighting in place of a plurality of independent lights,lighting powered by a battery source such as car brake lights, lightingin constant use such as traffic lights, warning and or road signs whichmay be optionally flashing or otherwise active, visible or near infrared security lighting, pulse mode camera flash applications, and thelike; and for advanced technology applications such as solid stateconjugated polymers lasers, micro cavity LED's enabling modification ofwave length and the like; and for application in clothing for safety orfashion purpose.

In further aspect of the invention there is provided the use of anitrogen and/or sulphur containing polymer as hereinbefore defined as ann-type semi-conductor, for example for use in a semi-conductingelectronic device, such as a transistor, solar cell, a photodiode,diode, triode and the like.

In a further aspect of the invention there is provided the use of anefficient electron transporting and/or hole blocking polymer as acomponent of a semi-conductor or semi-conducting device as hereinbeforedefined. It is a particular advantage that the present inventionprovides a new range of efficient electron transporting polymers, theadvantageous properties of which were heretofore unknown, by means ofproviding the polymer substantially free of materials capable oftrapping negative charge or quenching luminescence as hereinbeforedefined.

In a further aspect of the invention there is provided a method for theproduction of a device as hereinbefore defined. A device may suitably beproduced by any technique as known in the art. Preferably a device ashereinbefore defined is producing by providing a first support layer ofdesired dimensions and coating this with successive component layers ashereinbefore defined. Preferably metallic component layers are depositedby electrolytic or reactive means or by evaporation. Preferably asemi-conducting polymer as hereinbefore defined is coated in a layer ashereinbefore defined by means of spin coating or dip-coating of thepolymer or a precursor thereof, and optionally curing, or by equivalenthigh precision technique such as electrochemical deposition orevaporation for providing a polymer film.

Preferably a device as hereinbefore defined comprises or is adapted tobe associated with means for mounting and operation thereof in knownmanner. It is a particular advantage that devices of the invention maybe viewed from a large range of angles simplifying their mounting, andgreatly improving their visibility. It is a further advantage that thedevices may be used to provide a rapid response for pulse operation.Devices may be used in AC or DC operation, with the hereinbeforementioned advantages of power consumption supply and brightness.

In a further aspect of the invention there is provided a method for theoperation of light emitting diode as hereinbefore defined. Suitably alight emitting diode as hereinbefore defined is operated in knownmanner, by applying voltage, causing a current to flow through thedevice. Advantageously a device as hereinbefore defined may beconveniently and reliably operated by virtue of the high externalquantum efficiency thereof.

The invention is now illustrated in non limiting manner with referenceto the following figures and examples.

FIG. 3 illustrates the construction of an LED device as known in theart;

FIG. 4 illustrates an LED comprising a device according to theinvention.

FIG. 5 illustrates a display screen comprising the device of FIG. 2.

FIG. 6 illustrates the current voltage—light output characteristic ofthe device of FIG. 4.

FIGS. 7 and 8 illustrate the light output—current characteristic ofdevices of FIG. 4 and devices of FIG. 3.

FIG. 9 illustrated the efficiency—thickness ratio characteristic of abilayer device of the invention.

The construction of a typical electrode/polymer/electrode device isillustrated in FIG. 3. A high work function electrode such an indium-tinoxide (ITO) (4.6 eV), deposited on a glass substrate (1) serves as theanode (2) and is semi-transparent at thickness of 7-10 nm. The precursorof polymer, such as PPV is deposited as a thin film on the electrodeand, converted into the conjugated material, thereby providingsemi-conducting polymer layer (3). A low work function metal, such ascalcium or aluminium is evaporated onto the polymer surface (3) by avacuum metal vapour deposition, and serves as the cathode (4).Electrical contacts are provided (5, 6) linking the anode (2) andcathode (4) to a suitable power supply (not shown). In order to achievecharge injection, high field strengths (of the order of 10⁶V/cm) arerequired, although with the use of thin films of the order of 100 nm,the forward bias voltage of the device can be as low as a few volts.

In FIG. 4 is shown the construction of an electrode/polymer/electrodedevice according to the invention. The device comprises the samecomponent parts as indicated for FIG. 3, optionally comprising aplurality of semi-conducting polymer layers. For the purpose ofillustration, FIG. 4 shows a layer (3), such as PPV as described withreference to FIG. 3, and additionally a semi-conducting polymer layer(7), such as polypyridine (Ppy) directly deposited from a solution ofPpy as the polymer, as a thin film on the pre-prepared PPV layer (3).The respective thickness of layer (3) and (7) is as hereinbeforedefined. For a first illustrative purpose layer (3) is of the order 120nm and layer (7) is the order of up to 40 nm, thereby having a thicknessratio of 3. For a second illustrative purpose layer (3) is of the orderof 90 nm and layer (7) is of the order of 50 nm, thereby having athickness ratio of 1.8. The device of FIG. 4 may be operated at fieldstrengths of the order of 10⁶V/cm in order to achieve charge injection.The forward bias voltage of the device may be of the order of 2-20 Vdepending on layer thickness.

In FIG. 5 is illustrated an outdoor display screen which comprises thedevice of FIG. 4 of which the layers are continuous throughout thecross-section area of the screen (8), or as repeating sub-units ofcontinuous layers (indicated as dotted lines). The front panel of thedisplay comprises a glass substrate layer (1) as hereinbefore describedfor FIG. 4, or a suitable transparent material having mechanicalproperties adapted to resist deformation and fracture, and having arefractive index suited for the designed light emission properties.

The device of FIG. 4, having a thickness ratio of 1.8 providing anexternal quantum efficiency of greater than or equal to 0.2% andbrightness of the order of 1100 Cd/m² in continuous wave operation, orbrighter in pulsed operation may be visible from a distance, dependingon the colour of visible light emission, detail and clarity of the imageor message being portrayed. This compares with an inferior range for asimilar image or message portrayed on a screen constructed of the deviceof FIG. 3, having an external quantum efficiency of the order of 0.01%or less and brightness of the order of tens of Cd/m².

EXAMPLE 1 Preparation of a Device of the Invention

A glass or quartz surface of thickness of the order of 1 mm, coated withITO to a thickness of 80 nm+/−15 nm was used as the support forcomponent layers of the device as hereinbefore defined, commerciallyavailable (Balzers 30 Ohms per square). PPV precursor polymer comprisingthe tetrahydrothiophenium (THT) leaving group as a solution in methanolwas spin coated to a thickness of 100 nm onto the glass/ITO substrate,and converted to PPV by heating at 250° C. for ten hours under dynamicvacuum. A Ppy layer comprising the polymer in solution prepared bydissolving Ppy powder in 97% formic acid was spin coated directly ontothe PPV layer to form a homogeneous thin film requiring no thermalconversion, and of a thickness of up to 100 nm+/−5 nm. The cathodecomprising aluminium was coated onto the Ppy layer by evaporation at apressure of 5×10⁻⁶ mbar, at a rate of 0.1-0.5 nms⁻¹.

Ppy powder used in the preparation of the device was prepared byconventional technique but modified with use of a nickel catalyst, incontrolled fashion. The product polymer obtained by this novel modifiedprocess was substantially uncontaminated by cations of salt componentsemployed in the catalyst preparation, such as zinc, which may operate asa trap or quench in disadvantageous manner as hereinbefore defined.

The purity of the polymer may be selected as appropriate, for examplethe photoluminescence quantum yield gives one measure of the purity ofthe material.

EXAMPLE 2 Electro Luminescence of a Device of the Invention

The device prepared according to Example 1 was connected to a suitablepower supply by electrical contacts placed at the ITO anode and the Alcathode. The device was operated at a turn-on voltage of 12V andcurrent-voltage and current-intensity measurements carried out. From thedata obtained and indicated in FIGS. 6 and 7, the external quantumefficiency was determined.

The results are shown in Table 1.

EXAMPLE 3 Preparation of a Device of the Invention

A device according to the invention was prepared using the method ofExample 1 but employing MEH-PPV as the emissive layer, prepared from theappropriate precursor.

EXAMPLE 4 Electro Luminescence of a Device of the Invention

The device prepared according to Example 3 was connected and operated asdescribed in Example 2. The results are shown in Table 1.

EXAMPLE 5 Electro Luminescence of a Device not according to theInvention

Comparative devices were prepared using known techniques, and ashereinbefore defined, comprising the device of FIG. 1, and the device ofFIG. 2 having inefficient layer thickness ratio. The process of Example2 was employed and the same measurements were conducted as described forExample 2 and external efficiency data were determined.

The results are shown in Table 1 and FIG. 7

TABLE 1 SEMI- CONDUCTING POLYMER EXTERNAL THICKNESS QUANTUM DEVICE(PPV/PPY RATIO) EFFICIENCY ITO/PPV/Ppy/Al 140 nm (1.8) 0.25%ITO/PPV/Ppy/Al 160 nm (1.66) 0.21% ITO/PPV/Ppy/Al 160 nm (0.33) 0.004%ITO/PPV/Al 140 nm (−) 0.004% ITO/PPV/Ca 120 nm (−) 0.01%ITO/MEH-PPV/Ppy/Al 250 nm (1.0) 0.04% ITO/MEH-PPV/AL 110 nm (−) 0.009%

It will be appreciated that the accuracy of layer thickness is afunction of the materials employed, the techniques for preparation andthe skill and apparatus available, however layer thickness of spincoated polymers obtained directly from solution as hereinbefore definedare generally prepared to an accuracy of +/−5 nm. Deviations inthickness across a device as hereinbefore defined may contribute toinhomogeneity of the electric field in the device whereby some reductionin efficiency, uniformity and lifetime may be observed.

From Table 1 it is clear that:

(i) the bilayer device with PPY and PPV with optimised thickness ratio(top row) is 60 times more efficient than a similar device without thepolypyridine layer (4^(th) row)

(ii) the bilayer device is also much more efficient than a single layerdevice with a calcium contract (5^(th) row)

(iii) bilayer MEH-PPV devices (6^(th) row) are a factor of 4 moreefficient than similar devices without the polypyridine layer (7^(th)row). The results demonstrate a substantial improvement in efficiencywith both PPV and MEH-PPV as the emissive layers, which may be expectedto apply also with a wide range of existing (or future) luminescentmaterials. Without being limited to this theory the magnitude ofincrease efficiency is thought to be proportional to the barrier toelectron injection for example in MEH-PPV.

With reference to Table 1 it will be apparent that the appropriateselection of respective layer thickness for a given device comprisingpositive and negative charge conducting polymers has a dramatic effecton the efficiency of the device. Moreover it will be appreciated thatpolymers, previously considered to be essentially non-conducting, may berendered conducting in admirable manner by careful control of thesynthesis thereof, in particular with reference to the synthesis of Ppy.

With reference to FIGS. 7 and 8, in which the slope of the figures fordevices according to the invention and not according to the invention,is proportional to the external quantum efficiency, the distinction isvery clearly indicated in terms of the dramatically higher externalquantum efficiency of the device according to the invention, and asillustrated in FIG. 4, compared with the device not according to theinvention, and as illustrated in FIG. 3.

The upper FIG. 7 shows the intensity-current density characteristics ofbilayer and single layer diodes. The solid line shows the behaviour of abilayer PPV/PPY diode of thickness 140 nm and a PPY:PPY thickness ratioof 1.8. The dashed line shows the results for a 120 nm thick PPV devicewith calcium electrodes. The dotted line shows the results for a 140 nmthick PPV device with aluminium contracts. The external quantumefficiency of each diode is proportional to the slope of each curve. Thebilayer diode has an efficiency of 0.25%, much higher than theefficiency of 0.01% for the PPV/Ca diode and 0.004% for the PPV/Aldiode.

The lower FIG. 7 shows the current density-field (line) andintensity-field (symbols) characteristics of the bilayer diode. Thecurrent density shows typical diode field dependence.

The FIG. 8 shows intensity-current density characteristics of bilayerand single layer MEH-PPV/PPY and MEH-PPV diodes. The bilayer diode hasan efficiency of 0.04%, much higher than the single layer diodeefficiency of 0.009%.

The FIG. 9 shows efficiency vs thickness ratio characteristics forbilayer diodes. The figure shows that maximum efficiencies are achievedfor an optimum ratio of thickness, which coincides with balancedelectron and hole transport which is thought to coincide at a junction,within the layer capable of light emission by luminescence.

Further advantages of the invention will be apparent from the foregoing.

What is claimed is:
 1. Device for use in efficient light emissioncomprising a plurality of component layers of which a first outer layeris used for electron injection, a second opposing outer layer is usedfor “hole” injection, and at least two intermediate layers arrangedtherebetween are used for charge semi-conduction, wherein theintermediate layers to comprise semi-conducting polymer materials coatedin layers by spin coating or dip coating of the polymer or a precursorthereof, and comprise at least one semi-conductor polymer used forelectron transport and/or hole blocking, and at least one semi-conductorpolymer used for hole transport and/or electron blocking, wherein the atleast one semi-conducting polymer used for electron transport and/orhole blocking comprises polymer selected from a nitrogen and/or sulphurcontaining polymer which is partially or substantially conjugated,wherein ratio of thickness of semi-conducting polymer layers are in theregionof 0.1-10, the least thickness layer representing the leastefficient charge semi-conductor.
 2. Device as claimed in claim 1 whereinthe migration and coincidence of electrons and/or “holes” is adapted tobe manipulated by means of controlling layer thickness, to create aboundary region of coincidence of negative and positive charge carriers,hereinafter a junction, positioned relative to the first and secondouter layers in an emissive region in manner to provide enhancedbrightness and/or external quantum efficiency.
 3. Device as claimed inany of claim 1 wherein the at least one semi-conducting polymer adaptedfor electron transport and/or hole blocking is selected from aconjugated polycyclic in which at least one nitrogen and/or sulphur is aheteroatom comprised within a conjugated heterocyclic system.
 4. Deviceas claimed in claim 1 wherein the junction coincides with or bridges theinterface between two semi-conducting layers, which are of differentcharge semi conduction nature.
 5. Device as claimed in claim 1 whereinat least one charge semi-conduction layer or a component thereof, isadditionally capable of light emission by electroluminescence.
 6. Deviceas claimed in claim 5 wherein the at least one charge semi-conductionlayer or a component thereof, which is additionally capable of lightemission by electroluminescence, is substantially coincident with thejunction of coincidence of charge carriers.
 7. Device as claimed inclaim 1 wherein the ratio of thickness of semi-conducting layers are inthe region of 0.15-9, the least thickness layer representing the leastefficient charge semi-conductor.
 8. Device as claimed in claim 1 whereina component layer adapted for electron injection comprises a metalselected from calcium, magnesium, gold, or aluminium, optionally as anadmixture with suitable agents.
 9. Device as claimed in claim 1 whereina component layer adapted for hole injection is comprised of a holeinjection material, selected from a metal, alloy, or semi-conductor suchas indium tin oxide (ITO), tin oxide, as a transparent conductor, PEDOT,polyaniline or like polymer, gold, and admixtures thereof with suitableagents.
 10. Device as claimed in claim 1 wherein a component layeradapted for electron transport and/or hole blocking is comprised of an-type conducting nitrogen and/or sulphur containing polymer ashereinbefore defined in claim 1 or 2 selected from the group consistingof polypyridine (Ppy), polyalkylpyridines, polypyrimidines,polyalkylpyrimidines, polythiazoles, polyalkylthiazoles, derivativessuch as fluorinated derivatives, analogues and functional equivalentsthereof.
 11. Device as claimed in claim 1 wherein electron transportand/or hole blocking semi-conductor polymer is substantiallyuncontaminated by electron quenching or trapping species and holetransport and/or electron blocking semi-conducting polymer issubstantially uncontaminated by hole quenching or trapping species. 12.Device as claimed in claim 11 wherein the electron transport and/or holeblocking semi-conductor polymer is of high purity, is in substantiallycation free from, wherein cations deriving from polymerisation reagentemployed or the preparation of such reagents are substantially absent assynthetic residue from the polymer structure.
 13. Device as claimed inclaim 1 wherein the hole transport and/or electron blocking polymer iscomprised of any p-type conducting material, which is a conjugatedorganic molecule, dye or dye-doped polymer systempolyparaphenylenevinylene (PPV),poly(2methoxy-5-(2′-ethyl-hexyloxy)-p-phenylenevinylene) (MEM-PPV),cyano PPV, poly(p-phenylene), poly(alkylthiophene), derivatives,monomers, oligomers, analogues and functional components thereof. 14.Device as claimed in claim 1 wherein comprises a further supporting,sealing or protective layer.
 15. A device as claimed in claim 1 for usein a light emitting display (LED) of any desired surface area forportable or fixed purpose.
 16. A method for the production of a deviceas claimed in claim 1 comprising providing a first support layer ofdesired dimensions and coating this with successive component layers ashereinbefore defined.
 17. Method for the operation of a light emittingdiode comprising a device as defined in claim 1, comprising applying avoltage and causing a current to flow through the device.
 18. Device asclaimed in claim 1 wherein ratio thickness of semi-conducting layers is0.3-5, the least thickness layer representing the least efficient chargesemi-conductor.
 19. Device as claimed in claim 13 wherein the holetransport and/or electron blocking polymer is comprised ofpolyparaphenylenevinylene (PPV),poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylenevinylene) (MEM-PPV),cyano PPV, poly(p-phenylene), poly(alkylthiophene), derivatives,monomers, oligomers, analogues and functional components thereof.